Mostly we calculate from 6:00 AM local Sunrise time and 6:00 PM Sunset time. If the Sun rise and Sunset time is differerent then you may calculate from that time .Each choghadia period is one hour and thirty minutes (1.30 Hours) so you may calculate from your sunrise time , eight,(8) choghadia are day time and eight (8) choghadia are night time so that total choghadia are 16 in all day and night time.
